1. Technical Field
This invention relates to valves and, more particularly, to sealing valve actuator assemblies.
2. Background Art
Valve assemblies commonly include a valve housing having an inlet and an outlet forming a fluid passage therebetween. The valve assembly further includes an actuator assembly comprising a stem assembly and a driver mechanism connected to the stem assembly. Generally, the actuator assembly operates to move or drive the stem assembly, thereby sealing off or permitting the fluid transfer.
Most actuator assemblies penetrate the valve housing. Sealing the actuator assembly to the valve housing is an important design consideration to prevent leakage of valve fluid between the valve housing and the ambient environment. A typical method of sealing includes securing a rubber gasket or O-ring between the valve housing and the actuator assembly.
Some actuator assemblies include a pliable cup-shaped diaphragm, or boot, to provide sealing between the valve fluid and the ambient environment. Although the boot provides some degree of sealing, the stem still must penetrate the boot, and therefore an additional seal is required.
Another valve sealing problem is encountered with valves having auxiliary actuators. If the primary valve is rendered inoperable, an auxiliary actuator may be used to operate the valve. Like the primary actuator, the auxiliary actuator assembly must be sealed to the valve housing.
Rubber gaskets or boots do not provide a hermetic seal under all conditions. The gasket or boot may leak when the internal pressure of the valve fluid is significantly higher than the ambient pressure, for example with servo valves. Rubber gaskets or boots are also prone to degradation over time and may be inadequate if the fluid to be sealed is environmentally harmful.
Another shortcoming with the valve designs of the prior art is that the stem assembly penetrates the boot. This configuration requires an extra seal, which can degrade and leak over time with valve usage and cycling.
Therefore, there is a need for a better sealing arrangement for valves.